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NBSIR 84-2892

Responses to Questions By the

General Accounting Office

Related to Construction of the

Sunshine Skyway Bridge

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Structures Division

Gaithersburg, Maryland 20899

June 1984

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U.S. DEPARTMENT OF COMMERCE

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NATIONAL BUREAU OF STANDARDS

34-2392

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NBSIR 84-2892

RESPONSES TO QUESTIONS BY THE

GENERAL ACCOUNTING OFFICE

RELATED TO CONSTRUCTION OF THE

SUNSHINE SKYWAY BRIDGE

Nicholas J. Carino

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Structures Division

Gaithersburg, Maryland 20899

June 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. EniMt Amblar. Dkeetor

ABSTRACT

The General Accounting Office (GAO) requested the assistance of the National

Bureau of Standards In the Investigation of the construction of the new

Sunshine Skyway Bridge In Florida. Speclflcal I y, GAO desired answers to

questions related to the fol lowing: 1) the formation of cracks In the main

piers of the bridge span; 2) the materials used In the concrete mixtures; and

3) the procedures used In the placement of concrete In the drl I led shaft

foundations. The objective of the GAO Inquiry Is to determine the

reasonableness and validity of the positions taken by the Florida Department

of Transportat Ion on each of the concerns expressed by a number of Individuals

In connection with the bridge construction. This report provides answers to

the questions and provides explanations for each answer.

1 1 I s cone I uded that the cracks I n the p I ers will not affect the structure I capacity of the piers. However, the cracks should be sealed as a precaution

against the Intrusion of additional seawater, which may cause localized

corrosion of the reinforcement at any existing breaks In the epoxy coating on the reinforcing steel. The coarse aggregates that were used appear to have

resulted In a good quality concrete which should be durable. Final 1 y, there

Is no basis for concern with regard to the Integrity of the drl I led shafts.

Keywords: Bridges; concrete; construction; cracking; Investigation
TABLE OF CONTENTS

Page

ABSTRACT i i i

1.0 INTRODUCTION 1

2.0 BACKGROUND 2

3.0 RESPONSES TO QUESTIONS 7

4.0 CONCLUSIONS 23

5.0 REFERENCES 24

I V
1.0 INTRODUCTION

On April 11, 1984, the National Bureau of Standards (NBS) received a request

from Florida Congressmen C. W. Bl I I Young, Sam Gibbons and Andy Ireland to

provide technical assistance to Investigators of the General Accounting Office

(GAO) In their review of the construction of the new Sunshine Skyway Bridge.

On April 16, 1984, a representative from GAO met with NBS structural engineers

to discuss the nature of the GAO Investigation and to present copies of

technical documents that had been gathered by GAO. In addition, GAO presented

a I 1st of specific questions related to the construction of the Sunshine

Skyway Bridge. The NBS engineers performed a preliminary review of the

documents and questions. On May 1, 1984, NBS responded to the Congressmen’s

request and agreed to assist the GAO In Its Investigation.

The underlying objective of GAO’s questions Is to determine the reasonableness

and validity of the position taken by the Florida Department of Transportation

(FDOT) on the concerns raised by the Congressmen. This report presents the

responses to the questions and explanations for the answers. The responses

are based on the study of documents provided by GAO plus other technical

references related to the questions. Additional Information was obtained In a site visit during May 16-18, 1984.

The GAO made siml I ar requests for assistance to the U.S. Corps of Engineers

and the Bureau of Reclamation. During the course of this work, NBS maintained

contact with the Corps of Engineers and the Bureau of Reclamation. The

concerns and the GAO questions were discussed. In general terms, among the

representatives of the three agencies. Responses to the questions, however,

were Independently prepared by each agency. Draft copies of these responses

were distributed among the agency representatives for Information purposes.

The responses provided in this report represent the views of the National

Bureau of Standards.

The fol lowing chapter provides background Information to assist the reader who

Is unfamiliar with the circumstances leading up to the GAO Investigation.

1
2.0 BACKGROUND

The new Sunshine Skyway Bridge Is a 21,878 ft long structure being bul It at

the mouth of Tampa Bay to replace the existing bridge connecting St.

Petersburg with Bradenton, Florida. The twin-bridge structure being replaced

original ly consisted of a northbound span and a southbound span. On May 9,

1980, a tanker col I Ided with the southbound span and a portion of the bridge

fell Into the bay. The collapsed portion was never repaired and the

northbound span continues to carry traffic In both directions. A decision was made to build a modern replacement structure, and In 1982 construction

commenced for the new Sunshine Skyway Bridge.

The new Sunshine Skyway features a 3-span, cable-stayed bridge providing 175

ft. of vertical clearance over the shipping channel. As Indicated In Fig. 1,

the main span will be 1200 ft. long with 540-ft. side spans. The bridge will

be built using the technique of precast segmental construction. In which

precast concrete box girder segments are I If ted Into place and made continuous

by post-tens I on I ng.

The Sunshine Skyway Bridge Is being built under three separate contracts;

-Contract 1 to construct the main pier foundations for the cable-stayed

bridge,

-Contract 2 to construct the high level approach spans and the cablestayed bridge, and

-Contract 3 to construct the low level approach spans (cal led the trestle

portion)

.

The questions raised by GAO deal with the construction covered by Contract 1.

The cable-stayed bridge has two towers to support the single plane of stay

cables. Below the roadway, the ma I n supports cons I st of dua I elliptical

columns resting on reinforced concrete piers. Each pier Is supported on

reinforced concrete drilled shafts. Figures 2 and 3 show the cross sections

of the piers. The piers are composed of three structural elements:

1 ) the top slab,

2) the hoi low cone section, and

3) the bottom ring.

The top slab carries the load from the columns to the hoi low cone, and the

cone transfers loads to the bottom ring. The ring transfers the load to the

dr I I led shafts.

Figure 3 shows the dates of concrete placement In the various portions of the

north and south piers (the bridge runs approximately In the north-south

direction). Shortly after the cone of the north pier was placed, a representat I ve of FOOT detected cracks In the cone. A representative of the

designer Inspected the cracks and Informed FOOT that the cracks would not

Impair the structural capacity of the piers. The designer also Informed FOOT

that the* cones should be fl I led with water.

On October 20, 1983, a FOOT diver submitted a report to FOOT on his Inspection

of the exterior and Interior of the piers. The report noted that cracks

existed In both cones, some of which extended the f u I I cone height of 14 ft.

2
In addition. It was reported that 'feome of the cracks had efflorescence stains

and "I Ight corrosion stains".

In early November, 1983, a local television station reported on the existence

of cracks within the cones, and widespread media coverage of the situation

ensued. On November 11, 1983 Congressman Young dove Into one of the hoi low

cones to personally Inspect the cracking. After the Inspection, Young stated

that an Investigation should be carried out to explain the reason for the

cracks. On November 12, FOOT decided to begin pumping water out of the cones

to permit more detailed Inspection of the cracks and the associated

efflorescence deposits.

On November 17, 1983, Congressmen Young, Gibbons, and Ireland submitted a request to GAO to Initiate an Investigation Into the construction of the new Sunshine Skyway Bridge. On AprI I 11, 1984, the Congressmen requested that

NBS provide technical assistance to GAO In answering their concerns about the

cracks In the cones, the materials used In the concrete, and the procedures

used In placing the dr I I led shafts. The same request was also subm I tted to

the Bureau of Reclamation and to the Corps of Engineers.

A rep resent at I ve from GAO met with NBS eng I neers to d I scuss GAO*s techn leal

questions. Documents relating to the technical concerns, which were obtained

from FOOT, were given to the NBS engineers. These documents, along with

others subsequently supplied by FOOT, were used to help formulate the answers

to the questions. The following Is a I 1st of the types of documents that were

provided;

- Inspection reports on the cracks In the cones

- Structural drawings and project specifications

- Concrete mixture proportions

- Results of laboratory tests on concrete mixtures

- Placement records and cylinder strength results

- Report of the concrete consultants

- Excerpts from the Massachusetts Institute of Technology (MIT)

report deal Ing with the Sunshine Skyway

- Proposals for Instrumentation of the bridge

- Copies of media coverage of the construction

Additional Information was obtained from a site visit during May 16-18, 1984.

The visit Included an Inspection of the Interior of the cones and meetings

with FOOT personnel. Representatives from the Bureau of Reclamation and the

Corps of Engineers also participated In the site visit. The representatives

from the three agencies cooperated In al I aspects of the site visit, and on May 18, they held a meeting to discuss. In general terms, their proposed

responses to GAO's questions.

In total, GAO posed 14 questions. The questions related to three major

concerns:

1) the significance of the cracks In the cones of the piers,

2) the materials used In the concrete, and

3) the procedures used In placing the drilled shafts.

The fol lowing chapter provides the answers and explanations for the answers to

the questions.

3
4
PIER 5

Figure 2

Cross

sections

of

main

piers

for

cable-stayed

bridge
TOP

SLAB

roCO m

00

6

Figure

3

Dates

of

concrete

placement

in

various

sections

of

piers
3.0 RESPONSES TO QUESTIONS

QUESTION 1.1

In mass concrete pours are cracks, such as those described in the packet

provided you, to be expected as normal shrinkage cracks?

-RESPONSE

Yes, because in mass concrete It is difficult to prevent the formation of

"shrinkage cracks". Efforts are usual ly taken to control the spacing and

width of the cracks that form.

EXPLANATION

"Shrinkage cracks" result when concrete attempts to decrease In volume but Is restrained from doing so. Because of the restraint, tens! le strains (and

stresses) are Induced in the concrete. When the Induced strains exceed the

strain capacity of the concrete, cracks develop and extend Into the concrete.

Crack extension ceases where the Induced strains are less than the strain

capacity. To prevent the formation of these cracks It Is necessary to control

the Induced tensile strains.

The fol lowing is a brief explanation of the probab I e source of the Induced

tensile strains In the cone sections of the piers. Concrete gains its

strength from chemical reactions between the cement and the water. These

chemical reactions give off heat and the heat of hydration of the cement Is a quantitative measure of the total heat evolved up to a given age. For

example, a heat of hydration of 70 cal /gram at 7 days means that each gram of

cement will have evolved a total of 70 calories of heat after reacting with

water for seven days. The heat of hydration wl I I cause the temperature of the

concrete to rise. The maximum temperature attained by a given structural

member wi I I depend on many factors which are discussed In detail In Ref. (1).

One of the Important factors Is how readily the heat of hydration Is

dissipated through the member Into Its surroundings. A member having a large

ratio of surface area to volume can dissipate the heat of hydration more

readily than a member with a sma I I ratio. The temperature rise would

therefore be higher for the member with the low surface to volume ratio. The

term "mass concrete" refers to members whose dimensions are such "that

measures should be taken to cope with the generation of heat and attendant

volume change to minimize cracking (1)." Note that this definition says

"minimize" not "prevent" cracking.

The Initial temperature rise of the concrete causes It to expand, and If the

expansion Is restrained compressive stresses are Induced. However, at early

ages the stiffness (Young*s modulus of elasticity) of the "...concrete Is so sma I I that compressive stresses Induced by the rise In temperature are

Insignificant even in zones of restraint and can be Ignored (1)." After a concrete member reaches Its peak temperature. It begins to cool, causing a tendency for contraction. "If a concrete member has a uniform tendency to

contract but is restrained at Its base or at an edge, cracking will Initiate

at the base or restrained edge where the restraint Is greatest and wl I 1 progress upward or outward until a point Is reached where the stress Is

Insufficient to continue the crack (1)."

During the construction of the piers, concrete was placed In the cone

sections after the bottom rings were built. For the north pier the cone was

7
placed 65 days after the bottom ring, and for the south pier the age

difference was 43 days. When the cone concrete for each pier started to <~oo! the bottom rings prevented the cones from free I y contract i ng. As a resu 1 1,

tensile strains (and stresses) were induced in the cones, and when these

strains surpassed the capacity of the concrete, cracks developed. As

indicated in the above quote from Ref. (1), the cracks would be expected to

start at the bottom of the cone wa I I and propagate toward the top. Ref (1)

further states:

"...u n re i n forced concrete wa I Is or slabs, subject to base

restra i nt, w I I I u 1 1 imate I y atta i n cracks through the f u I I b I ock

height spaced in the neighborhood of 1.0 to 2,0 times the height of

the block,”

According to the cracking survey suppi led by FOOT, the average lateral spacing

between the vertical cracks In the cone of the north pier was approximately 13

ft. and for the south pier the average was 11 ft. The vertical height of the

cone walls Is 14 ft. Thus the average crack spacing Is in agreement with the

above statement.

Note that the above quotation concerning crack spacing refers to unreinforced

concrete. In reinforced concrete members, reinforcing steel is usually

provided to control the width of "shrinkage cracks". In the case of the

cones, page 9 of the structural drawings indicates that about 50 #8 bars are placed in the circumferential direction. This corresponds to a reinforcement

ratio of about 0.24$, which is in close agreement with the requirement of a minimum of 0.25$ horizontal reinforcement in wal Is to control cracking due to

temperature and shrinkage as given in section 14.2.10 of AC I 318-77 (2).

In order to have prevented the formation of thermal ly Induced cracks it would

have been necessary to have used an internal piped cool ing system to prevent a large temperature rise of the concrete, or perhaps the cone cou Id have been

cast in several smal ler sections. The first alternative would probably have

been an expensive proposition and the second alternative would probably have

Introduced serious construction problems.

In summary, the formation of cracks In the cones of the pier foundation was to

be expected considering the massive nature of the structure and the

construction sequence that was used.

8
QUESTION

Would the type of concrete mix used result In cracks?

P£SP.QN^£

The character I st Ics and proportions of the Ingredients In the concrete are

some of the factors that affect the problem of control I Ing cracks due to

restrained volume changes.

EXPLANATION

The Interrelationships between a concrete mixture and the problem of

controlling cracking are discussed In detail In Ref, (3); the key points are summarized In the f o I lowing discussion. The two measures for control I Ing

cracking of concrete are: 1) to produce a concrete having the best cracking

resistance (tensl le strain capacity); and 2) to control the factors which

produce tensile strain in the concrete.

The tensile strain capacity of concrete Is directly proportional to Its

tensile strength and Inversely proportional to Its modulus of elasticity. In addition, when the tensile strain Is gradual ly applied, a high creep rate for

the concrete Is helpful In controlling cracking. For these reasons Ref. (3)

makes the fol lowing statement concerning aggregate selection to achieve a concrete with a large strain capacity:

"Where severe I sources of aggregate are aval I ab I e economica I I y,

preference should be given to that which yields best crack

resistance; usual ly this wll I be a crushed material of low thermal

expansion and low modulus of elasticity.”

Thus the Florida limestone, which was used In the concrete and which Is known

to have a relatively low el ast Ic modu I us, may have been of some benefit In

control I Ing the formation of cracks In the cone.

The other measure for control I Ing cracking Is to minimize the Induced tensl le

strains by limiting volume change and restraint. Volume change can be

control led by limiting the maximum temperature rise of the concrete at early

ages. This can be achieved by:

-using the lowest quantity of cement that Is consistent with

strength and durability requirements

-using cement with low heat of hydration at early ages

-using pozzolans as a replacement for some of the cement

-using the lowest possible amount of mixing water

-precooling the concrete during Its production

Control I Ing restraint Is a more difficult matter as a balance must be attained

between minimizing restraint and keeping the construction simple.

The project specifications cal I for the fol lowing:

-use of low-heat cement (ASTM Types II or IV)

-use of 20 to

"50% fly ash (a pozzolan) replacement for cement

-use of water-reducing admixtures

-use of Ice for the mixing water

In conclusion, the project specifications Included reasonable steps for

control ling shrinkage Induced cracking In the main piers.

9
QUESTION 1.5

FOOT acknowledges that some of the cracks may go a I I the way through the cone

wa I I. What Is the significance of cracks that go al I the way through the r wail?

RESPONSE

The cracks are not significant in terms of the structural capacity of the

piers but they may be of significance in terms of long-term performance.

EXPLANATION

First, the phi losophy of reinforced concrete design must be recognized. In designing a reinforced concrete member to safely carry a given external load,

it is assumed that the portion of the member subjected to tensi le stress is

cracked and sufficient steel is provided to carry the necessary tensi le

loading. Hence, the presence of cracks in reinforced concrete members does

not necessarily mean that structural capacity has been affected. To determine

the significance, if any, of cracks in the cones it is necessary to consider

the structural action of the cones and the relationship between crack

orientation and load paths through the piers.

According to information provided by FOOT; "The cone acts primarily as a massive compression member... There are some local and bending effects also in

the cone." This means that the cone walls carry an inclined axial force plus

a bending moment. The structural drawings indicate a large number of #14 and

#18 bars running in the vertical direction of the cones. These represent the

reinforcement assisting the concrete In carrying the load from the

superstructure down to the foundation. The circumferentia I bars in the cones are sma I 1 er (#8 bars) and the area of this reinforcement is 0.24$ of the

concrete area of a vertical section through the cone. As explained in the

response to question 1.1, the amount of circumferential steel meets the

minimum requirements for the control of shrinkage cracking. The adequacy of

the circumferential steel to resist tensile stresses due to the external load on the piers depends on the magnitude of the circumferential stresses that

may develop under the action of that load. It is our understanding that FOOT

is in the process of performing a stress analysis of the piers. The results

of that analysis should Indicate whether the circumferential steel Is adequate

to carry any tensi le stresses that may develop under service loads.

The cracks may have some significance in terms of the long-term performance of

the piers. Specifical I y, the cracks may permit localized corrosion on some of

the reinforcing steel. Although the bars are epoxy-coated, there are probably

breaks in the coating which may have been produced during the placement and

consolidation of the concrete. In addition, there could also be breaks that were produced during the placement of the steel which were not seen during the

patching of the coating. If any of these defects In the coating happen to lie

near the cracks, there Is the possibl I Ity of local Ized pitting due to the

presence of seawater. While such localized corrosion presents no short-term

problems, it is not certain what may happen in the long-term. The basic

problem Is that epoxy-coated bars are relatively new products and we do not

have sufficient historical records to determine the effects of smal I defects

on long-term performance.

During the site visit to the piers it was observed that some of the cracks

that were leaking water contained brown deposits along with lighter colored

deposits. In one crack In the south pier the brown material could be seen

10
coming out of the crack at the bottom of a core hole. The Information that

has been provided to us does not give a precise explanation of the nature of

the brown colored deposits. It has not been shown conclusively that the brown

color Is not due to rust. An attempt should be made to secure samples of only

the brown material so that It can be chemical ly analyzed. The results of the

analysis should be compared with the analysis of deposits from other sources

that are known to contain corrosion products. It then should be possible to

determine whether the brown material Is rust. It Is also suggested that the

efflorescence deposits should be cleaned so that the nature of any new

deposits can be monitored.

In summary, the vertical cracks In the cones do not appear to have any

significance In terms of the structural capacity of the piers, but they may

have some significance In terms of long-term durability.
QUESTION 1 .4

Are FDOT's plans to seal the inside cracks a reasonable solution, that is,

will seal ing the cracks make the water within the cone walls inert and thus

not a threat to the rebars?

RESPONSE

FOOT should re-examine its current plan for sealing the cracks, giving careful

consideration to the primary objective of any repair that is undertaken. It has not been adequately explained how sealing the cracks on one surface will

make the water "inert".

EXPLANATION

As discussed in the response to question 1.3 the cracks pose an unknown factor

with respect to future performance of the epoxy-coated steel. Thus It would

be best to take a conservative approach and undertake remedial measures. The

technique which Is finally implemented should be consistent with the overa I I objective of the repair.

First, consideration should be given to the requirements for active corrosion

of bare steel In concrete, which are as fol lows:

-a supply of oxygen,

-an electrically conductive path in the concrete, and

-depassivation of the steel.

If one or more of these items is eliminated, corrosion ceases.

Chloride ions present in seawater depass ivate bare steel and may also increase

the electrical conductivity of saturated concrete. Water-fi I led cracks in the

concrete provide an access for chloride ions and dissolved oxygen to the

steel. Because of diffusion, it is not necessary for the water to flow

through the cracks in order for additional chloride Ions and oxygen to migrate,

to the steel. Thus the objective of the repair method should be to keep

additional chlorides or oxygen from reaching any bare spots on the epoxycoated bars.

The repair method that is used should result in the filling of the existing

cracks so that further penetration of chloride ions and oxygen is prevented.

Because the repair is not intended to provide structural strength across the

cracks, it is not necessary to use an epoxy adhesive type of material. The

feassibi I ity of using non-structura I materials that have been developed

exclusively for the purpose of seal ing cracks should be investigated.

Therefore, it is suggested that careful consideration be given to the

technique that will be used to repair the cracks in the cone wa I Is. It is

felt that the proposal to seal the cracks only on the interior surfaces of the

cones is not the most appropriate approach, as this wi I I not exclude the

future migration of chloride ions and oxygen into the cracks.

12
gU£ST..Ig.N.

When do or can shrinkage cracks become cracks of structural significance?

RESPONSE

Shrinkage cracks are of structural significance In a concrete structure when

they reduce the load capacity or service I Ife of the structure or If they

prevent the structure from serving Its Intended function.

EXPLANATION

As was mentioned In the discussion of question 1,3, the design of a reinforced

concrete structure assumes that the concrete will crack and reinforcing steel

Is provided to carry the tensile forces In the members. If a shrinkage crack

develops In that portion of a member that will be subjected to tens I le loading

while In service. It Is of no concern because the reinforcing steel will carry

the tension. If a shrinkage crack develops In that portion of a member that

will be subjected to compressive loading while In service. It Is of no concern

because the crack will close up under the compressive load. Thus shrinkage

cracks In a properly designed reinforced concrete member are not structural ly

significant provided that cracking does not result In premature deterioration

of the structure, such as through corrosion of the reinforcing steel.

There are many factors to consider when addressing the question of the

significance of cracks on the durability of a concrete structure. One of the

most Important is the exposure conditions. Depending on the severity of the

exposure conditions different crack widths can be tolerated. The fol lowing

general guide for tolerable cracks widths has been adopted by AC I Committee

224 (3);

Exposure Condition Tolerable Crack Width

Dry air or protective membrane 0,016 In Humidity, moist air, soil 0,012 In Deicing chemicals 0,007 In Seawater and seawater spray; wetting

and drying 0,006 In Water retaining structures 0,004 In

Of the 18 crack width measurements reported by FOOT for the Interior of the

north and south cones, six exceeded 0,006 Inches but none exceeded 0.010

Inches, While the reported crack width measurements are not necessarily the

maximum crack widths along each crack, they Indicate that most of the existing

cracks might be considered tolerable even for seawater exposure.

13
QUESTION ZA

Does your agency have a position regarding the use of Florida limestone?

response

The National Bureau of Standards has not carried out research on the use of

Florida limestone In concrete, and thus has no position regarding the use of

this aggregate.

QUESTION 2^

Is FDOT's assertion that control of the mix and other materials offsets the

limestone characteristics a reasonable one?

RESPONSE

Based on the reported concrete cylinder strengths for the piers. It Is

concluded that the control procedures adopted by FOOT have accommodated three

of the four stated deficiencies of the limestone aggregates.

EXPLANATION

The concrete consultants* report Identified four problems with using Florida

aggregates:

1) variability of geologic formations In different parts of the state,

2) tendency for excessive degradation during handling, leading to the

production of excessive fine material,

3) high and variable value of absorptivity, and

4) low strength and stiffness.

Because of the first three characteristics the consultants felt that by using

Florida aggregate It wou Id be difficult to contro I the variability of the

properties of the finished product, that Is, the concrete.

The fol lowing table was developed based on the cyl Inder strengths reported by

FOOT for the various placements In the two piers:

E 1 ement

Requ 1 red

Strength

(psi

)

Average

Measured

Strength

(psI

)

Standard

Deviation

(psi

)

Coefficient

of Variation

(%)

N. Seal 3000 5670 357 6.3

S . Sea 1 3000 4040 216 5.3

N. Shafts 3400 4990 524 10.5

S. Shafts 3400 4720 491 10.4

N^ Bot. Ring 4200 5560 382 6.9

S. Bot. Ring 4200 5135 258 5.0

N. Cone 4200 5120 322 6.3

S. Cone 4200 5425 366 6.7

N. Top Slab 4200 5680 403 7.1

S. Top Slab 4200 5450 324 5.9

14
According to ACI Standard 214-77 (4), for general construction testing, a standard deviation less than 400 psi corresponds to excel lent control and a standard deviation between 400 and 500 psI represents very good control. Thus

It Is clear that the procedures Implemented by FOOT have produced concrete

with consistent strength properties.

Concerning the question of low aggregate stiffness. It Is not possible to

compensate for this characteristic. However it was found that the elastic

modu I us of concrete made with the Brooksv I I I e aggregate was reported to be

about 4,000 ksl. This measured value agrees with that predicted by the ACI

formula which considers the unit weight and compressive strength of the

concrete. Thus It Is concluded that the concrete made with the Brooksvllle

aggregate has the magnitude of elastic modulus expected for Its strength and

density.

15
QUESTION 2^

If the recommended Georgia and Alabama aggregate had been used, would the

quality of the concrete be measurably better? How?

response

It Is uncertain whether the use of the Georgia or Alabama aggregates would

have. In I tse I f , resu I ted I n concrete of significantly h Igher qua I I ty than

that obtained with the Florida limestone.

EXPLANATION

First, the meaning of "quality concrete" needs to be defined. Quality

concrete Is concrete which has the required properties In both Its fresh and

hardened state, which Is economical, and which has an acceptable degree of

variability. From this definition. It Is seen that what Is, and what Is not,

"quality concrete" depends on the specifics of each project. The production

of quality concrete requires both the use of acceptable materials and the use

of sufficient controls to assure that the materials are correctly handled and

proportioned.

Judging from the average values and standard deviations of the reported

cylinder strengths. It would have to be concluded that the concrete used In the piers Is of good quality. It Is uncertain that a better quality concrete

would have resulted If the Georgia or Alabama aggregates had been used. The

harder aggregates might have resulted In si Ightly higher compressive strengths

for the same amount of cement (cement factor). This could have resulted In using a lower cement factor which could have reduced the temperature rise In the concrete p I acements. On the other hand, had the harder aggregates been

used. It Is poss Ib I e that a d I f f erent contro I program wou I d have been used

which could have produced concrete of higher varlabi I Ity than was attained

with the limestone. With the harder aggregates, the resulting concrete would

have had a higher elastic modulus and reduced creep potential. While this may

have been of some benefit for the design of the superstructure. It would not

have been a benefit In control ling cracking In the cones due to Induced

tensile strains. Thus It cannot be concluded with certainty that use of the

Alabama and Georgia aggregates would have produced a better qua I Ity concrete.

16
OUfS.TI .QN. Zx4

Can the actual aggregate properties of creep and shrinkage be Incorporated

Into the design as MIT suggests and are the creep and shrinkage factors for

concrete made with Florida limestone known?

RESPONSE

Yes, the actual creep and shrinkage properties of the concrete can be

Incorporated Into the design, and the necessary data are available.

EXPLANATION

For prestressed concrete structures, a key element In their design Is the

consideration of the time-dependent deformations due to creep and shrinkage.

These have to be considered, for example, to predict the deformation of the

structure and to determine the required level of Initial prestressing forces.

A textbook on the subject of prestressed segmental bridges states that (5):

”Re liable data on the potential of the mix In terms of strength

gain, creep, and shrinkage performance should be developed to serve

as the basis for Improved design parameters.”

Thus for design purposes, good engineering practice requires knowing the

properties of the concrete to be used In the structure.

Creep and shrinkage data of concrete made with Florida aggregates were

developed for FOOT by the Waterways Experiment Station of the Corps of

Engineers. In addition, representatives of FOOT have stated that Construction

Technology Laboratories of the Portland Cement Association and the University

of Florida are also generat I ng data on concrete with the aggregates used In

the bridge construction. It Is also our understanding that FOOT Is requiring

the designer to update the original design by taking Into account the

available creep and shrinkage data.

In summary, the creep and shrinkage properties of the concrete being used to

build the the bridge can and should be Incorporated Into the final design of

the superstructure.

17
m3im

What Is the relationship. If any, between the decision to use Florida

limestone and the cracks In the concrete?

besponse

There Is no significant relationship between the use of the Florida limestone

and the formation of the cracks In the cone walls.

EXPLANATION

The explanation for the causes of the cracks In the cone walls and the effect

of the concrete mixture on the formation of cracks was given In the

discussions of questions 1,1 and 1,2. To repeat, cracks occur when the

Induced tensile strains resulting from restrained volume change exceed the

tensile strain capacity of the concrete. In the case of the cones, the volume

change was brought about by the temperature rise of the mass concrete and the

restraint was due to the presence of the bottom ring. Had Florida aggregates

not been used, there probably would not have been any significant changes In these two factors and, a I I other conditions being the same, the cracks would

sti I I have developed.

16
QUESTION U.

Should the concrete mix formula used In the main pier foundations result In quality and durable concrete?

RESPONSE

Based on the available Information, the concrete used In the piers Is

considered as being of good quality and It should perform well.

EXPLANATION

As mentioned In the discussion of question 2,3, quality concrete results when

acceptable materials are used and when the materials are handled and

proportioned In a proper manner. In this particular situation, several

specific actions were taken by FOOT to ensure a quality concrete:

1) use of low-heat cement with I Imitations on the contents of

alkalies and tricalclum alumlnate,

2) use of entrained air and a water reducing admixture,

3) use of Ice for mixing water to lower the Initial concrete

temperature,

4) use of fly ash to replace some of the cement,

5) use of a low water-cement ratio,

6) limiting of the approved sources for coarse aggregate, and

7) requiring water sprlnkl Ing of the coarse aggregates to control

absorption.

The results of tests by Construction Technology Laboratories Indicated that

the limestone aggregate could be considered Innocuous with respect to

reactivity with alkalies In cement. Thus It Is unlikely that there would be

any durability problems because of the a I ka 1 1 -aggregate reaction.

The resu I ts of the cy I I nder strength tests, as d I scussed I n the response to

question 2.2, suggest that very good to excel lent quality control was used to

produce the concrete. In summary, there are no Indications that the concrete

Is of poor qua I Ity or that It wi I I have poor durabi I Ity.
QUESTION

Would the five "above normal" steps taken by FOOT enhance the concrete?

RESPONSE

The five steps, plus others that were Implemented, are positive approaches for

enhancing the expected performance of not only the concrete but also the

structure.

EXPLANATION

It Is our understanding that the five steps In question are as follows:

1) use of cement having low heat of hydration, low alkali content

and resistance to sulfate attack,

2) use of fly ash to replace some of the cement,

3) use of Ice,

4) use of epoxy coated reinforcing steel, and

5) use of a penetrant sealer on exposed concrete surfaces.

As discussed In the response to question 3.1, the first three Items are

helpful In producing durable concrete. The use of epoxy coated bars and the

sealer are helpful In minimizing the possibility of rebar corrosion. Another

deterrent to corrosion Is the 4 Inches of concrete cover used on the steel In the piers.

In summary. It appears that FOOT has taken reasonable steps towards producing

a durable concrete and a durable structure.

20
mSTIPN

Is the 2-hour pour time a general rule-of-thumb time and Is a 28-hour set time

feasible with the mix used?

response

To our knowledge, there Is no rule-of-thumb time I Imit on the placing of a drilled shaft. The Information provided us Indicates that the room

temperature Initial setting time of a concrete mixture similar to that used

for the shafts would be expected to be greater than 6 hours.

EXPLANATION

The probable reason for putting a time limit on the placement of the concrete

In the dr 1 1 led shafts Is so that there would be no possibility of developing

cold Joints within the shafts. A cold joint Is the discontinuity that

develops If the next layer of concrete Is placed on a previous layer that has

already undergone setting, thereby precluding a union of the concrete In

adjacent layers. Hence the Important parameter Is not the elapsed time for

the placement of the entire shaft but rather It Is the elapsed time between

the p I acement of adjacent I ayers.

The setting time of a concrete mixture Is dependent on many factors. Some of

the more Important ones Include the concrete temperature, the Initial

consistency, and whether admixtures are used. At room temperature the Initial

setting time of concrete would be expected to be within 4 to 6 hours.

However, the concrete used In the shafts would be expected to have a setting

time longer than 4 to 6 hours for the fol lowing reasons:

-an admixture was used,

-the Initial concrete temperature was lower than room temperature, and

-the slump was high.

Based on the Information provided us. It is unlikely that the setting time of

the concrete In the shafts was as high as 28 hours. The long setting times

reported by FOOT were for concrete mixtures with a set retarding admixture.

However, the admixture used in the shafts, referred to as LL-819, does not

appear to be a set retarding admixture. Manufacturer’s I Iterature states that

"concrete with admixture LL-819 sets at a rate comparable to plain

concrete..." It Is further stated that "Admixture LL-819 Is recommended for

use In al I concrete where normal setting characteristics are required or

desired." Test data provided on the performance of LL-819 Indicate that at a dosage rate of 2-1/2 fl oz/100 lb cement and for concrete having a 2-3/4 Inch

s I ump, the Initial setting time Is 6 h 10 m compared with 5 h for a concrete

without the admixture. The concrete In the shafts contained the admixture at

a dosage of 5 oz/100 lb of cement, which would be expected to cause additional

retardation of setting. For this reason and because the concrete had a high

slump and an initial temperature below the standard used to determine setting

time. It Is concluded that the concrete In the shafts would have had an

initial setting time In excess of 6 hours. However, It Is unlikely that the

Initial setting time would have been as high as 28 hours.

21
QUESTION 3.4

What Is the significance of the 18 pourings exceeding the 2-hour specification

(no pouring exceeded 4 hours) In I Ight of the 28-hour set time?

response

There Is no significance to the fact that the elapsed times for the placement

of 18 of the shafts exceeded the 2-hour specification limit.

EXPLANATION

As discussed In the response to question 3.3, the real concern Is the elapsed

time between the placement of adjacent layers In a shaft, rather than the time

to place the entire shaft. According to the data provided us, none of the 18

shaft placements In question exceeded 4 hours, and 15 of them were placed

within 2-1/2 hours.

Because the concrete contained an admixture which had some retarding effect

and because the concrete was placed at a lower than normal temperature. It Is uni Ikely that Initial setting had begun In the shafts even after 4 hours.

Thus there would be no adverse effects to those shafts that surpassed the 2-

hour I Imit.

22
4.0 CONCLUSIONS

Based on our study of the Information provided us by GAO and the additional

Information obtained from FOOT, the fol lowing conclusions are drawn with

respect to the three concerns addressed by NBS:

1. Si gnificance of cracking In pler^

The cracks pose no Immediate concern with respect to the structural capacity

of the ma I n p I ers. The cracks are probab I y the resu 1 1 of restra I nt to the

thermal shrinkage due to the temperature rise and subsequent cool I ng that

occurred In the massive concrete placements. The Florida Department of

Transportat Ion took reasonable steps In preparing the Job specifications to

minimize the temperature rise of the concrete placements. It would have

required a much more comp I Icated construction procedure to have total ly

el Imlnated the formation of thermal shrinkage cracks. However, since the

cracks can permit the Intrusion of seawater. It Is prudent that they be

sealed. The Intrusion of seawater, which contains chloride Ions and dissolved

oxygen, can lead to localized corrosion of the reinforcement at the location

of any defects In the epoxy coating,

2. plater I a I ? in Ibn concrete

The reported cylinder strength results for Contract 1 show that the concrete

produced with the Florida I Imestone aggregates had low varlobi I Ity and average

strengths we I I above the design strengths. Thus there Is no question that the

concrete Is adequate from a strength point of view. Since the aggregates are

Innocuous with respect to reactivity with alkalies In the cement and since fly

ash and a low water cement ratio were used, the concrete Is expected to be

durable. Concerning the creep and shrinkage behavior of the concrete, FOOT Is

making efforts to obtain additional data that can be Incorporated In the final

designs of the superstructure. In addition, there are plans to Instrument the

completed structure In order to monitor Its time-dependent behavior.

3. Ergggdurgg .usgd in placing concrete in ihS. drilled shafts

The concern that some of the shafts may be weakened because the concrete

placement took longer than the 2-hour specification limit Is unfounded.

Considering that the Important elapsed time Is that which occurs between the

placement of adjacent layers and not that which occurs between the start and

completion of the entire shaft placement, and considering that the Initial

setting time of the concrete was probably In excess of 6 hours. It Is highly

uni Ikely that cold Joints developed.

23
REFERENCES

1, "Effect of Restraint, Volume Change, and Reinforcement on Cracking of

Massive Concrete," ACl 207.2R-73, Manua I of Concrete Practice. American

Concrete Institute, Detroit, 1981, pp 207-39 to 207-64,

2, "Building Code Requirements for Reinforced Concrete, " ACl Standard 318-

77, American Concrete Institute, Detroit, 1977, pg 63.

3, "Control of Cracking In Concrete Structures," ACl 224R-80, Concrete

International, Vo I, 2, No, 10, October 1980, pp 35-76,

4, "Recommended Practice for Evaluation of Strength Test Results of

Concrete," ACl Standard 214-77, American Concrete Institute, Detroit,

1977, pg 7.

5, Podo I ny, W. and Mu I I er, J.M., Construction sM Design ai Prestressed

Concrete Segmenta I Bridges. John Wiley & Sons, New York, 1982, pg 466,

24
NBS-114A (REV. 2-BO) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2892

2. Performing Organ. Report No 3. Publication Date June 1984

4. TITLE AND SUBTITLE

Responses to Questions by the General Accounting Office

Related to Construction of the Sunshine Skyway Bridge

5. AUTHOR(S)

Nicholas J. Carino

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The General Accounting Office (GAO) requested the assistance of the National

Bureau of Standards In the Investigation of the construction of the new

Sunshine Skyway Bridge In Florida. Specifical I y, GAO desired answers to

questions related to the fol lowing: 1) the formation of cracks In the main

piers of the bridge span; 2) the materials used In the concrete mixtures; and

3) the procedures used In the placement of concrete In the dril led shaft

foundations. The objective of the GAO Inquiry Is to determine the

reasonableness and validity of the positions taken by the Florida Department

of Transportation on each of the concerns expressed by a number of Individuals

In connection with the bridge construction. This report provides answers to

the questions and provides explanations for each answer.

1 1 I s cone I uded that the cracks In the piers w I I I not affect the structural

capacity of the piers. However, the cracks should be sealed as a precaution

against the Intrusion of additional seawater, which may cause localized

corrosion of the reinforcement at any existing breaks In the epoxy coating on the reinforcing steel. The coarse aggregates that were used appear to have

resulted In a good quality concrete which should be durable. Final I y, there

Is no basis for concern with regard to the Integrity of the dr!

I

led shafts.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon

bridge; concrete; construction; cracking; investigation

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS). Springfield, VA. 22161

14. NO. OF

PRINTED PACES

28

15. Price

$8.50

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